Observation of Preformed Electron-Hole Cooper Pairs

TL;DR

This paper reports the first experimental observation of preformed electron-hole Cooper pairs in a semiconductor, revealing a new state of matter at high electron-hole densities and low temperatures.

Contribution

It provides the first evidence of electron-hole Cooper pairs in a semiconductor, demonstrating a crossover from excitons and plasma to this new paired state.

Findings

Observation of stimulated emission indicating preformed Cooper pairs

Identification of crossover regimes in electron-hole density and temperature

Evidence supporting BCS-like pairing in semiconductors

Abstract

Electrons and holes in a semiconductor form hydrogen-atom-like bound states, called excitons. At high electron-hole densities the attractive Coulomb force becomes screened and excitons can no longer exist. Bardeen-Cooper-Schrieffer theory predicts that at such high densities co-operative many-body effects can at low temperatures induce a bound state, an electron-hole Cooper pair, comparable to an electron-electron Cooper pair in a superconductor. Here we report the first observation of preformed electron-hole Cooper pairs in a semiconductor. By measuring stimulated emission from a dense electron-hole gas in ZnO, we have explored both the crossover from the electron-hole plasma to the preformed Cooper-pair regime, and the crossover from the exciton to the preformed Cooper-pair regime.